Controlled release of acid in a wellbore penetrating a carbonaceous formation

ABSTRACT

A method for controlled release of acid in a subterranean carbonaceous formation is achieved by placing in the openhole section of a wellbore within the formation a crosslinked acid of a volume substantially equal to the volume of the openhole section, then using tubing placed within the openhole section to circulate into the crosslinked acid a composition containing a breaker for the crosslinked acid at a matrix rate while removing the tubing from the openhole section, breaking the crosslinked acid while the tubing is removed, closing the backside and squeezing the uncrosslinked acid into the subterranean carbonaceous formation. In one non-limiting embodiment the openhole section may be substantially horizontal.

TECHNICAL FIELD

The present invention relates to methods and compositions for controllably releasing acid in a subterranean carbonaceous formation, and more particularly relates, in one non-limiting embodiment, to methods and compositions for controllably releasing acid in a subterranean carbonaceous formation that more evenly distributes acid into a subterranean carbonaceous formation.

BACKGROUND

In the exploration and production of hydrocarbons, such as crude oil and natural gas from subterranean formations, matrix acidizing is the treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sand-stone formations, the acid reacts with soluble substances in the formation matrix to enlarge the pore spaces within the matrix. In carbonate formations, the acid dis-solves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of the reservoir fluids. Matrix acidizing operations are preferably performed at a high rate, but at treatment pressures below the fracture pressure of the formation; this lower rate is also known as the “matrix rate”. The matrix rate may be sufficient to create worm-holes past or through the damaged area of the near wellbore region. This procedure enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation. Acid treatments are also used to clean up near-wellbore formation damage after drilling. Such damage may inhibit the subsequent production of reservoir fluids.

However, there are particular challenges to acid treatments in deviated holes, that is, wellbores intentionally drilled in a direction away from vertical, which challenges increase as the wellbore becomes substantially horizontal. “Substantially horizontal” is defined herein as being within ±10° of horizontal, alternatively as being within ±5° of horizontal. “Directional drilling” may be understood to be where the departure of the wellbore from vertical exceeds about 80°. Some horizontal wells may be designed such that after reaching true 90° horizontal (from vertical) the wellbore may in fact start drilling upward. In such cases, the angle past 90° is continued, as in 95°, rather than reporting it as a deviation from vertical, which would be 85°. It is known that because a horizontal well typically penetrates a greater length of a reservoir, it may offer significantly improved production compared to a vertical well.

Additionally, openhole carbonates are difficult to clean up with acid after drilling because the acid will create its own path of least resistance and be used up in only a relatively small portion of the wellbore rather than throughout the target interval. The result is less than optimum stimulation. Various methods have been employed using foams and gels to divert acid over the entire wellbore with some success.

Nevertheless, it would be desirable if methods could be devised to distribute the acid more uniformly over all or at least a greater proportion of the wellbore.

SUMMARY

There is provided, in one non-limiting form, a method for controlled release of acid in a subterranean carbonaceous formation having a wellbore with at least one openhole section having a bottom or toe, where the method includes placing in the openhole section a crosslinked acid in a volume substantially equal to the volume of the openhole section. The method additionally includes using tubing placed within the openhole section, circulating into the openhole section a composition at a rate (such as a matrix rate), where the composition comprises a breaker for the crosslinked acid beginning proximate to the bottom or toe of the openhole section while removing the tubing from the openhole section. The method further includes breaking the crosslinked acid while the tubing is removed, to give uncrosslinked acid, closing the backside, and squeezing the uncrosslinked acid into the subterranean carbonaceous formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a substantially horizontal openhole section of a wellbore after loading with a crosslinked acid;

FIG. 2 is a schematic illustration of the substantially horizontal openhole section of the wellbore of FIG. 1 after running into the openhole to the toe with tubing;

FIG. 3 is a schematic illustration of the substantially horizontal openhole section of the wellbore of FIG. 2 illustrating the pumping of breaker solution through the tubing and into the crosslinked acid;

FIG. 4 is a schematic illustration of the substantially horizontal openhole section of the wellbore of FIG. 3 illustrating flushing uncrosslinked acid into the formation while pulling the tubing out of the hole while flushing;

FIG. 5 is a schematic illustration of an alternate embodiment of the method showing pumping a composition containing an acid and a breaker through the tubing of the wellbore of FIG. 2 and into the crosslinked acid; and

FIG. 6 is a schematic illustration of the wellbore of FIG. 5 showing that as pressure builds up, pulling the tubing back to treat the next interval.

It will be appreciated that FIGS. 1-6 are schematic and that the various elements are not necessarily to scale or proportion, and that many details have been removed or simplified for clarity, and thus the invention is not necessarily limited to the embodiments depicted in the Figures.

Before the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseol-ogy and terminology used herein are for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION

A method has been discovered that permits the controlled release of acid in a wellbore that is penetrating a carbonaceous formation which more evenly or more uniformly distributes acid along the wellbore. Generally, the method involves using tubing to introduce acid and a breaker (optionally separately or essentially simultaneously) into crosslinked acid already placed within the wellbore, while removing the tubing.

In more detail, the method involves in one non-limiting embodiment, placing into a wellbore a volume of crosslinked 15 wt % HCl (without an internal breaker) equal to the wellbore volume across a producing interval (see FIG. 1). The crosslinked acid is substantially equal to the volume of the openhole section. By “substantially equal” is meant+/−25 vol % of the openhole section, alternatively +/−15 vol % of the openhole section, and in a different, non-limiting embodiment, +/−10 vol % of the openhole section.

Using tubing (jointed or coiled) and starting at the bottom of the openhole interval (or the toe, if it is a substantially horizontal wellbore) (see FIG. 2), the method additionally involves circulating, at matrix rate (a pump rate below that required to fracture the formation), a weak HCl (3-5 wt %) solution containing a small amount of fluoride solution while essentially simultaneously pulling the tubing out of the hole (see FIG. 3). The fluoride solution will break the zirconium crosslinking of the crosslinked HCl and release the HCl for immediate reactivity. This method would thus allow the acid to be placed and reacted uniformly throughout the interval. After the crosslinked fluid has been broken, the backside of the hole would be closed and the method would further involve squeezing the 15 wt % HCl into the formation (see FIG. 4). By the “backside” is meant the annulus surrounding a production tubing string, above the production packer. This would provide the bulk of the acid volume to be placed and reacted uniformly through the entire interval. An alternative, non-restrictive procedure would be to pump a diverting system (gelled by a crosslinked polymer or a viscoelastic surfactant) of about 15 to 20 wt % HCl solution, containing the fluoride breaker. This fluid would not only allow the crosslinked HCl to be uncrosslinked and become reactive, but could be pumped into the formation until the pressure built up from the viscosity increase of the spending viscoelastic acid solution approaches the fracturing rate (see FIG. 5). At this time, the tubing position could be moved (tubing withdrawn) and a new interval of crosslinked acid uncrosslinked and the sequence repeated until the entire interval has been treated (see FIG. 6).

In another alternative embodiment herein, the flushing fluid or fluid used to break the crosslinked acid may be an acid carrying the breaker or breaker mechanism; and within this embodiment the acid could be neat or the acid could contain a viscoelastic surfactant additive. In this version, if the fluid was carried away into one portion of the wellbore more than into another portion of the wellbore, this flushing fluid could provide some extra diversion in the formation to prevent these losses.

In still further detail, shown in FIG. 1, is a schematic illustration of a wellbore 10 having a substantially horizontal openhole section 12 after loading the openhole section 12 with a crosslinked acid 14, shown by the shaded portion, in the direction of the arrows 13 using tubing 16 at the top of the openhole section 12. It is not necessary that the openhole section 12 be substantially horizontal, or even that it be in a deviated hole. However, in some non-limiting embodiments, it is expected that the method described herein will provide the greatest advantage in openhole sections of deviated wells, and particularly in substantially horizontal openhole sections, since these are the intervals that are particularly difficult over which to distribute acid uniformly or evenly. In one non-limiting embodiment, the crosslinked acid has an absence of an internal breaker.

In one non-limiting embodiment, the crosslinked acid is crosslinked hydrochloric acid (HCl), where the crosslinking of a polymer is accomplished with zirconium ions. In one non-limiting embodiment, this may be a zirconium crosslinked polymer with HCl in the proportion of from about 7.5 independently to about 28 wt %, alternatively from about 10 independently to about 20 wt %, in a different embodiment from about 10 independently to about 15 wt %, and in another non-limiting embodiment about 15 wt %. As used herein the term “independently” with respect to a range means that any lower threshold may be combined with any upper threshold to provide a suitable alternative range.

Other crosslinked acids may be used, but crosslinked HCl is expected to be the crosslinked acid most commonly used. “Crosslinked HCl” is more precisely a crosslinked polymer that includes HCl, for instance a crosslinked water-soluble polymer, such as polyacrylamide, AMPS (2-acrylamido-2-methylpropane sulfonic acid), or an aqueous fluid gelled by a suitable viscoelastic surfactant (VES), such as those described in U.S. Pat. No. 7,723,272, e.g. amine oxides or amidoamine oxides and the like (incorporated herein by reference in its entirety), and the like and combinations thereof. Crosslinking agents include, but are not necessarily limited to zirconium ions, chromium ions, aluminum ions, iron ions, titanium ions, and the like and combinations thereof. Alternatively, crosslinked acetic acid or formic acid may be used, alone or together with crosslinked HCl. In one non-limiting embodiment, these should all be aqueous solutions.

Shown in FIG. 2 is a schematic illustration of the substantially horizontal openhole section 12 of the wellbore 10 of FIG. 1 after running into the openhole to the toe 18 (or bottom) with tubing 16.

Shown in FIG. 3 is a schematic illustration of the substantially horizontal openhole section 12 of the wellbore 10 of FIG. 2 illustrating the pumping of breaker solution, as shown by the arrows 19 through the tubing and into the crosslinked acid 14. In one non-limiting embodiment the breaker solution is a weak acid solution containing a relatively small amount of fluoride breaker to break the crosslinked acid, such as by disrupting the zirconium ion crosslinks of the polyacrylamide. The acid in the weak acid may be the same acid used in the crosslinked acid, in a non-limiting instance, HCl. One non-restrictive definition of “weak acid” in this case is from about 3 independently to about 5 wt %; alternatively from about 3.5 independently to about 4.5 wt %.

The breaker may be any suitable breaker for the crosslinked acid used. In the case of a fluoride breaker for zirconium crosslinked polyacrylamide, suitable soluble salts may be used, which may include, but are not necessarily be limited to, sodium bifluoride, calcium fluoride, or any other fluoride salt where the fluoride release can be controlled, and the like and combinations thereof. As the sodium bifluoride is consumed by the zirconium, more is released. The proportion or amount of fluoride breaker varies depending on well temperature; if the well is relatively hotter, less is required; if the well is relatively colder, more is required. In general, the amount or proportion of fluoride in the fluoride breaker ranges from about 0.01 wt % independently to about 0.15 wt % of the fluoride; alternatively from about 0.025 wt % independently to about 0.125 wt % of the fluoride.

The method further involves, as shown in FIG. 4 in a schematic illustration of the substantially horizontal openhole section 12 of the wellbore 10 of FIG. 3, closing the backside of the well (the annulus is closed at the wellhead) and flushing uncrosslinked acid, indicated by the arrows 21, propelled by flushing fluid (arrows 23) into the formation 20 while pulling the tubing 16 out of the hole 10 while flushing. It should be understood that the flushing fluid 21 is introduced as the tubing 16 is removed or pulled relatively gradually out of the hole 10. The flushing fluid may be water or another type of aqueous solution. The operations of flushing and pulling the tubing occur essentially simultaneously. By “essentially simultaneously” is meant that these processes occur at approximately the same time although they do not have to occur precisely simultaneously. For instance, it may be imagined that the introduction or injection of flushing fluid and pulling the tubing may occur continuously, and/or smoothly, and overlap in time. Alternatively, the introduction or injection of flushing fluid and pulling the tubing may occur intermittently or periodically or non-periodically, but still within the same time frame. The introduction or injection of flushing fluid and pulling the tubing may occur alternatingly, in a regular or non-regular sequence. In one non-limiting embodiment, these procedures may take place over a time period from about one-half hour independently to about 8 hours; alternatively from about 1 hour independently to about 6 hours; the total time from introduction of flushing fluid to the removal of the tubing.

Shown in FIG. 5 is a schematic illustration of an alternate embodiment of the method. This embodiment again involves a wellbore 10 having a substantially horizontal openhole section 12 after loading the openhole section 12 with a crosslinked acid 14, shown by the shaded portion, between the toe 18 and the heel 22 of the wellbore 10. However, in this embodiment, the tubing 16 is already extended through the wellbore 10 to the toe 18. In this embodiment, a composition 24 containing an acid and a breaker is pumped through the tubing 16 of the wellbore 10 and into the crosslinked acid 14. This composition 24 may be understood to be an in situ diverting system. The crosslinked acid is uncrosslinked (the gel or viscosity is reduced by removing the crosslinking agent), and the uncrosslinked acid and the additional acid build up viscosity and are pushed into a first interval 26. In one non-limiting embodiment, the acid and the breaker used may the acids and the breakers previously discussed in the proportions previously mentioned. In one non-restrictive instance, the in situ diverting system 24 may have from about 15 wt % independently to about 20 wt % HCl and a fluoride breaker in the proportions previously mentioned. This system would not only allow the crosslinked HCl to be uncrosslinked and become reactive, but may be pumped into the formation 20 until the pressure built up from the viscosity increase of the spending viscoelastic acid solution squeezes or pushes the acid into the interval 24 of formation 20. As defined herein, “squeezing” means the careful application of pump pressure to force a treatment fluid into a planned treatment zone. At this point, the tubing position may be moved (tubing end moved from toe 18 in direction of heel 22) and a new, second interval 28 adjacent to first interval 26 of crosslinked acid 24 is uncrosslinked and the sequence repeated until the entire substantially horizontal openhole section 12 has been treated (see FIG. 6). In other words, FIG. 6 is a schematic illustration of the wellbore 10 of FIG. 5 showing that as pressure builds up, the tubing 16 is pulled back to treat the next interval 28 in sequence. It will be appreciated that the amount of crosslinked acid 14 will diminish with each interval treatment away from the toe 18.

In both of the embodiments discussed, as the crosslinked acid 14 is uncrosslinked, or if a VES-gelled aqueous fluid is used, as the viscosity is reduced, the acid released by the breaking will go to the path of least resistance. In carbo-nate-containing formations, this is will be the path as the broken acid reacts with the carbonate. “Uncrosslinked acid” refers to the acid released as the gel is uncrosslinked (viscosity reduced) by removing the crosslinking agents from the polymer or reducing the viscosity of the VES, as applicable. By using the method described herein it is expected that the uncrosslinked acid is more evenly distributed into the subterranean carbonaceous formation as compared with a method where tubing is not removed during the method, that is moved out in the direction from the toe to the heel, in the case of a deviated well.

In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been described as effective in providing methods and compositions for distributing acid, particularly acid uncrosslinked from a crosslinked water-soluble polymer, over a subterranean carbonaceous interval. However, it will be evident that various modifications and changes may be made thereto without departing from the broader scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, specific fluids, gelled aqueous fluid, polymers, VESs, breakers, acids, steps and sequences of steps falling within the claimed parameters, but not specifically identified or tried in a particular composition or method, are expected to be within the scope of this invention.

As used herein, and in the claims, the words “comprising” and “comprises” is to be interpreted to mean “including but not limited to”.

The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. For instance, there may be provided a method for controlled release of acid in a subterranean carbonaceous formation having a wellbore with at least one openhole section having a bottom, where the method consists essentially of or consists of, not necessarily in this order, placing in the openhole section a crosslinked acid in a volume substantially equal to the volume of the openhole section, using tubing placed within the openhole section, circulating into the openhole section a composition at a rate, where the composition comprises a breaker for the crosslinked acid beginning proximate to the bottom of the openhole section, breaking the crosslinked acid to give uncrosslinked acid, removing the tubing from the openhole section, and squeezing the uncrosslinked acid into the subterranean carbonaceous formation.

Alternatively, there is provided a method for controlled release of acid in a subterranean carbonaceous formation having a wellbore with at least one substantially horizontal openhole section having a bottom, where the method consists essentially of or consists of placing in the substantially horizontal openhole section a crosslinked acid of a volume substantially equal to the volume of the substantially horizontal openhole section, in an absence of an internal breaker, using tubing placed within the substantially horizontal openhole section, circulating into the substantially horizontal openhole section a composition at a matrix rate, where the composition comprises a weak acid and a breaker for the crosslinked acid, beginning proximate to the bottom of the substantially horizontal openhole section while removing the tubing from the substantially horizontal openhole section, where the acid may be the same or different as the acid of the crosslinked acid and where the acid is present in the composition in a concentration between about 3 to about 5 wt %, breaking the crosslinked acid while the tubing is removed, to give uncrosslinked acid, closing the backside, and squeezing the uncrosslinked acid into the subterranean carbonaceous formation. 

What is claimed is:
 1. A method for controlled release of acid in a subterranean carbonaceous formation having a wellbore with at least one openhole section having a bottom or toe, where the wellbore has a backside, the method comprising not necessarily in this order: placing in the openhole section a crosslinked acid in a volume substantially equal to the volume of the openhole section; using tubing placed within the openhole section, circulating into the openhole section a composition at a rate, where the composition comprises a breaker for the crosslinked acid beginning proximate to the bottom or toe of the openhole section; breaking the crosslinked acid to give uncrosslinked acid; removing the tubing from the openhole section; closing the backside; and squeezing the uncrosslinked acid into the subterranean carbonaceous formation.
 2. The method of claim 1 where the crosslinked acid has an absence of an internal breaker.
 3. The method of claim 1 where removing the tubing from the openhole section and squeezing the uncrosslinked acid into the subterranean carbonaceous formation occur essentially simultaneously.
 4. The method of claim 1 where the crosslinked acid is crosslinked hydrochloric acid.
 5. The method of claim 4 where the proportion of acid in the crosslinked acid ranges from about 7.5 to about 28 wt %.
 6. The method of claim 1 where the breaker is a fluoride solution in an amount effective to substantially break the crosslinked acid, and where the fluoride solution comprises between about 3 to about 5 wt % fluoride.
 7. The method of claim 1 where the composition additionally comprises a weak acid solution where the acid is present in the composition in a concentration between about 3 to about 5 wt %, and where the acid may be the same or different as the acid of the crosslinked acid.
 8. The method of claim 6 where the acid in the weak acid solution is hydrochloric acid.
 9. The method of claim 1 where the composition is circulated at a matrix rate.
 10. The method of claim 1 where: circulating into the openhole section a composition; breaking the crosslinked acid to give uncrosslinked acid; removing the tubing from the openhole section; and squeezing the uncrosslinked acid into the subterranean carbonaceous formation; all occur essentially simultaneously.
 11. The method of claim 1 where the uncrosslinked acid is more evenly distributed into the subterranean carbonaceous formation as compared with a method where tubing is not removed during the method.
 12. The method of claim 1 where the openhole section is substantially horizontal.
 13. A method for controlled release of acid in a subterranean carbonaceous formation having a wellbore with at least one substantially horizontal openhole section having a bottom or toe, where the wellbore has a backside, the method comprising: placing in the substantially horizontal openhole section a crosslinked acid of a volume substantially equal to the volume of the substantially horizontal openhole section, in an absence of an internal breaker; using tubing placed within the substantially horizontal openhole section, circulating into the substantially horizontal openhole section a composition at a matrix rate, where the composition comprises a weak acid and a breaker for the crosslinked acid, beginning proximate to the bottom or toe of the substantially horizontal openhole section while removing the tubing from the substantially horizontal openhole section, where the acid may be the same or different as the acid of the crosslinked acid and where the acid is present in the composition in a concentration between about 3 to about 5 wt %; breaking the crosslinked acid while the tubing is removed, to give uncrosslinked acid; closing the backside; and squeezing the uncrosslinked acid into the subterranean carbonaceous formation.
 14. The method of claim 13 where the crosslinked acid is crosslinked hydrochloric acid.
 15. The method of claim 14 where the proportion of acid in the crosslinked acid ranges from about 7.5 to about 28 wt %.
 16. The method of claim 13 where the breaker is a fluoride solution in an amount effective to substantially break the crosslinked acid, and where the fluoride comprises between about 3 to about 5 wt % fluoride.
 17. The method of claim 13 where the acid in the composition is hydrochloric acid.
 18. The method of claim 13 where the uncrosslinked acid is more evenly distributed into the subterranean carbonaceous formation as compared with a method where tubing is not removed during the method.
 19. A method for controlled release of acid in a subterranean carbonaceous formation having a wellbore with at least one openhole section having a bottom or toe, where the wellbore has a backside, the method comprising: placing in the openhole section a crosslinked acid of a volume substantially equal to the volume of the openhole section and a composition; closing the backside; and essentially simultaneously: using tubing placed within the openhole section, circulating into the openhole section a composition at a rate, where the composition comprises a breaker for the crosslinked acid beginning proximate to the bottom or toe of the openhole section; breaking the crosslinked acid to give uncrosslinked acid; removing the tubing from the openhole section; and squeezing the uncrosslinked acid into the subterranean carbonaceous formation.
 20. The method of claim 19 where the crosslinked acid is crosslinked hydrochloric acid, and where the proportion of acid in the crosslinked acid ranges from about 7.5 to about 28 wt %.
 21. The method of claim 19 where the breaker is a fluoride solution in an amount effective to substantially break the crosslinked acid between about 3 to about 5 wt % fluoride. 